الفهرس 
2.2Pathogenicity of S.aureusOnly 14 pages are availabe for public view
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Abstract
Bacterial resistance to multiple antibiotics has become a global concern. Emergence of methicillin resistant Staphylococcus aureus (MRSA) strains as causatives of a lot of infections is a threat to the effectiveness of the most important treatment options available for clinicians. While a powerful arsenal of antibiotics was once capable of treating most infections, in early 1970s, clinical isolates of MRSA have emerged to numerous antibiotics usually used for their treatment.
In this study, phenotypic characterization of MRSA isolates by conventional methods and genetically in PCR technique, study the prevalence of mecA and blaZ genes within isolated MRSA.
One hundred and nineteen clinical samples from different hospitals in El-Sharkia Governerate, Egypt were randomly collected. Most of these samples were from wounds. Samples were collected from different wards and ICUs. Isolates were cultured on selective media such as blood-agar, mannitol salt agar and Baired-Parker agar. Sixty six isolates were identified as S. aureus, the identification was confirmed by Gram-stain, catalase and coagulase test.Antibiotic susceptibility test showed multidrug resistance to tested antibiotics with various extents. Resistance to oxacillin was found to be 77.3 % of all tested isolates, 48% were resistant to azithromycin, 100% to ceftazidime, 13.6 % to vancomycin, 76 % to meropenem, 68 % to cefotriaxone,100% to ampicillin, 77.2 % were resistant to cefotaxime, 45.4% were resistant to erythromycin,33 % to doxycyclin, 22.7 % were resistant to imipenem, 43.9 % were resistant to ciprofloxacin. The results demonstrated a high level of resistance among the isolates to most of the commonly used antibiotics. Minimum inhibitory concentration (MIC) of oxacillin for all S. aureus isolates was measured, most of the tested resistant staphylococci showed MIC value above the standard breakpoint for resistance (oxacillin MIC ≥ 4 μg/mL).
Genomic DNA was extracted from the bacterial isolates. PCR assay was carried out to screen for two important resistance genes (mecA and blaZ) associated with methicillin resistance and β-lactamase production causing β-lactam hydrolysis, respectively.
The PCR data showed that most of MRSA isolates 88% (n= 45) were found to harbor mecA gene. Only 11.7% (n=6) of the phenotypically oxacillin-resistant isolates were mecA-. On the other hand, blaZ gene was detected in 66.6% (n= 34) of the isolates. The coexistence of both mecA and blaZ genes were recorded in 56.8% (n= 29) of isolates, suggested that there is a cross-talk between the two regulatory systems, as bla regulators stabilize the mecA acquisition. While 33% (n=17) of the isolates were blaZ negative, It was also found that blaZ gene was present in 5 isolates that were mecA negative. The preservation of blaZ gene in MRSA isolates, even in the absence of mecA, could be implicated as “a first-line defense” against β-lactams. While the mecA- isolates containing blaZ gene, and still phenotypically resistant to oxacillin was suggested to be due to β-lactamase hyperproduction. Only one isolate encountered in this study was found not containing either blaZ or mecA, a rare phenomenon, may be due to the presence of a mutant gene or the presence mecC gene, a homologue of mecA, within a newly emerged cassette chromosome or there may be another unclear mechanism of resistance.
This work highlights the importance of using a combination of conventional and molecular detection in the diagnosis of MRSA isolates. The presence of β lactamase resistant MRSA strains lacking mecA gene suggests that phenotypically resistant MRSA could be misdiagnosed using molecular methods alone and provides evidence of alternative mechanisms for β-lactam resistance in MRSA. Our results suggest the possibility of the existence of unidentified mechanism of regulation involved in the transcriptional control of mecA gene in MRSA strains which is contradictory to the idea that in most clinical MRSA strains mecA gene is under the control of the bla regulatory genes